The adsorption of acrolein on a Pt (1 1 1): A study of chemical bonding and electronic structure

Applied Surface Science 263 (2012) 79–85

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The adsorption of acrolein on a Pt (1 1 1): A study of chemical bonding and electronic structure S. Pirillo a , I. López-Corral a , E. Germán b , A. Juan b,∗ a b

Instituto de Química del Sur (INQUISUR, UNS-CONICET) and Departamento de Química, Universidad Nacional del Sur, Av. Alem 1253, B8000CPB, Bahía Blanca, Argentina Instituto de Física del Sur (IFISUR, UNS-CONICET) and Departamento de Física, Universidad Nacional del Sur, Av. Alem 1253, B8000CPB, Bahía Blanca, Argentina

a r t i c l e

i n f o

Article history: Received 31 July 2012 Received in revised form 30 August 2012 Accepted 31 August 2012 Available online 7 September 2012 Keywords: Bonding Acrolein DFT Pt (1 1 1) Adsorption

a b s t r a c t The adsorption of acrolein on a Pt (1 1 1) surface was studied using ab-initio and semiempirical calculations. Geometry optimization and densities of states (DOS) curves were carried out using the Vienna Ab-initio Simulation Package (VASP) code. We started our study with the preferential geometries corresponding to the different acrolein/Pt (1 1 1) adsorption modes previously reported. Then, we examined the evolution of the chemical bonding in these geometries, using the crystal orbital overlap population (COOP) and overlap population (OP) analysis of selected pairs of atoms. We analyzed the acrolein intramolecular bonds, Pt (1 1 1) superficial bonds and new molecule surface formed bonds after adsorption. We found that Pt Pt bonds interacting with the molecule and acrolein C O and C C bonds are weakened after adsorption; this last bond is significantly linked to the surface. The obtained C Pt and O Pt OP values suggest that the most stable adsorption modes are 3 -cis and 4 -trans, while the 1 -trans is the less favored configuration. We also found that C pz orbital and Pt pz and dz2 orbitals participate strongly in the adsorption process. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The selective hydrogenation of the ␣–␤ unsaturated aldehydes such as acrolein, crotonaldehyde, or prenal is an important industrial process and much effort has been devoted to enhance the selectivity toward the unsaturated alcohol, which are important intermediates for synthesizing perfumes, flavoring and pharmaceuticals [1–5]. Usually hydrogenation of C C bond is kinetically and thermodynamically more favorable than that of C O group [6]. Thus, finding a catalyst with high selectivity and activity to allylic alcohols is a challenge. However, the knowledge of the interactions between such molecules and the metallic surface remains unclear even if the thermal activation of these aldehydes has been studied in order to identify the adsorption modes from the decomposition products. The rational design of new catalysts requires knowledge of the elementary mechanisms and, in a first step, knowledge of the adsorption modes of the ␣–␤ unsaturated aldehydes and the nature of the preferred adsorption sites. Quantum chemical calculations are an alternative tool to explore all the possible adsorption structures and to determine the most stable ones. Several catalytic routes have been used to enhance the yield of the unsaturated alcohol, including the use of boron and aluminum

∗ Corresponding author. Tel.: +54 291 4595142; fax: +54 291 4595142. E-mail address: [email protected] (A. Juan). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.125

hydrides and other reducing agents. Catalytic hydrogenation using transition metal-based catalysts has proven to be the most attractive way by far to carry out this process in industrial setting [7]. Nevertheless, only a few exceptional cases, mostly involving platinum-based catalysts have been reported for heterogeneous processes with reasonable selectivity toward the hydrogenation of the carbonyl group of unsaturated aldehydes and the formation of the corresponding unsaturated alcohol [2,5,8]. Acrolein (CH2 CH CH O), as the first member of ␣–␤ unsaturated aldehydes, is most difficult to be hydrogenated to unsaturated alcohol because there is no substituent on the C C group to hinder C C hydrogenation. Jesús and Zaera [8] studied the adsorption and thermal chemistry of acrolein on Pt (1 1 1) surfaces by temperatureprogrammed desorption (TPD) and reflection–absorption infrared (RAIRS) spectroscopies. They found that acrolein chemisorbs in an essentially flat geometry and interacts mainly via the carbonyl group. The authors also proposed that some trans to cis isomerization may take place upon interaction of the acrolein with the metal as well. Delbecq and Sautet [9] performed semiempirical (extended Hückel) calculations for acrolein on Pt and Pt surfaces. The same authors investigated the various possible adsorption structures for several ␣–␤ unsaturated aldehydes by means of density functional theory (DFT) calculations [10]. Loffreda et al. [11] presented the total energy calculations together with the frequency calculations for the stable

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chemisorption sites of acrolein on Pt (1 1 1). The authors optimized the geometry of s-trans and s-cis acrolein in the gas phase. They found that the energy difference between the s-cis and strans conformers of acrolein in the gas phase is 2.3 kcal/mol, which corresponds roughly to 98.0% of trans and 2.0% of cis at 300 K, compared with the 96.0 and 4.0% determined experimentally. Several adsorption modes for acrolein on the Pt (1 1 1) surface were considered according to the possible interaction of C C and/or C O bonds with the surface. They obtained new insights into the molecular adsorption modes by means of the comparison of the calculated electron energy loss spectroscopy (EELS) spectra with high-resolution energy loss spectroscopy (HREELS) measurements. These authors reported the DFT optimized geometries for various adsorption sites as a function of acrolein surface coverage. They found that for a wide range of coverage (between 0.08 and 0.2 ML, ML = monolayer) the 3 -cis form is one of the most stable site. For the 3 adsorption family, the acrolein C C bond interacts with the surface as for a di-␴C C form but moreover the O atom interacts also with a surface Pt atom. Both cis and trans conformers are stable leading to the respective 3 -cis and 3 -trans cases. In this work the density of states (DOS) of adsorbed acrolein and Pt (1 1 1) surface and the crystal orbital overlap population (COOP) curves between atoms and orbitals were calculated in order to analyze the adsorbate–substrate interactions. In particular, we focus our study on bonding interactions during molecular adsorption of acrolein molecules on the Pt (1 1 1) surface, including orbital occupation analysis.

2. Methodology 2.1. Theoretical methods We used DFT within the generalized gradient approximation (GGA) of Perdew–Wang 91 for exchange–correlation functional [12,13] implemented in the Vienna Ab-initio Simulation Package (VASP) code [14–17]. A plane wave basis set was used with an energy cutoff of 400 eV. In adsorption calculations, the Brillouinzone was sampled using 3 × 3 × 1 Monkhorst–Pack k-point mesh [18]. For the ionic relaxation, the forces acting on the ions were calculated using the Hellmann–Feynman theorem as the partial derivates of the free energy with respect to the atomic positions, including the Harris–Foulkes correction to the respective forces [19]. The DOS curves are plots of the number of orbitals per unit volume per unit energy, and are usually projected on the spherical harmonics inside the Wigner–Seitz (WS) sphere [20–22]. This assumption ignores the charges located into the interstitial area. An alternative approach to the analysis of atomic charges was proposed by Bader [23]. In the Bader’s method, the central point is to determine the region that corresponds to each atom by analyzing the topological properties of the charge density. In the present paper, an implementation of the Bader’s method developed by Henkelman et al. [24] was used. The DOS of both acrolein and Pt surface curves were calculated in order to analyze the adsorbate–substrate interactions. The bonding analysis was performed using the YAeHMOP package [25]. This method captures efficiently the essential orbital interactions. The overlap population density of states (COOP) curve is a plot of the overlap population weighted DOS versus energy. Integration of the COOP curve up to the Fermi level (EF ) gives the total overlap population (OP) of the specified bond [26]. Looking at the COOP, we may analyze the extent to which specific states contribute to a bond between atoms or orbitals. The OP shows the degree of bonding of two specified atoms. A positive number means a bonding interaction, while a negative number means an

antibonding interaction. When computing the DOS and COOP the system is divided into two fragments, consisting of the surface and adsorbate, respectively. This enables us to compare the changes between the bare surface, the adsorbate, and the composite adsorbed system [27].

2.2. Adsorption model There are several possible adsorption modes of ␣–␤ unsaturated aldehydes since each double bond can interact, separately or in combination, with the metal atoms. From experimental results it was suggested that acrolein isomerizes from trans to cis when it is adsorbed [8]. Delbecq and Sautet [10] proposed that the adsorption structures of adsorbed acrolein onto Pt (1 1 1) can be separated in four classes: interaction by the C C or C O bond (with possible di-␴ or ␲ structure, implying 2 or 1 metal atom, respectively), interaction by both bonds simultaneously (called 4 ), and interaction by the oxygen lone pair. The authors also considered a di-␴C C structure where the oxygen atom additionally interacts with the surface (3 -cis). This last adsorption mode is suggested by previous semiempirical calculations as an especially stable one [9]. The cited authors studied 10 structures in the case of a 1/9 ML coverage. They found that the most stable adsorption mode is the 3 -cis one where acrolein is in a cis conformation and is adsorbed through its C C bond, with a secondary interaction between the oxygen atom and the surface. The di-␴C C mode without this secondary interaction is 1.9 kcal/mol less stable that the previously mentioned. The 4 -trans mode is 0.4 kcal/mol less stable whereas the 4 -cis mode is farther apart (3.4 kcal/mol). The di-␴C O trans and the atop adsorption modes are just weakly bound to the surface. In conclusion, Delbecq and Sautet [10] suggested that at low coverage, two flat adsorption modes can coexist which both involve the whole molecules, the 4 -trans and the 3 -cis ones. Loffreda et al. [11] presented complementary results to those previously published [10]. However, in this case only seven adsorption geometries were investigated based on energy stability criteria [11]. These authors found that the 1 -trans adsorption mode, in which the molecule is bonded to the surface only by the O atom, is the less stable one and for all the coverages studied the top-trans site, is never competitive. On the other hand, three stable adsorption positions are possible for the 2 adsorption family (di-␴C C and di-␴C O forms). Both conformers (cis and trans) are stabilized by an interaction between their C C bond and two surface Pt atoms resulting in the 2 -cis and 2 -trans structures, respectively. The O atom does not interact with the surface. Both 2 forms have almost the same adsorption energy. These adsorption modes are not competitive for low coverage range. However, for high coverage range, both 2 -cis and 2 -trans are the most stable forms. The authors also studied the 3 adsorption family, in which acrolein C C bond interacts with the surface as for a di-␴C C form but moreover the O atom interacts also with a surface Pt atom. Both conformers (cis and trans) are stable, and for a wide range of coverage the 3 -cis form is one of the most stable adsorption sites. Finally, they proposed the so-called 4 site family, in which both C C and C O bonds interact with the surface Pt atoms resulting in flat adsorption forms. These authors found that whatever the coverage the 4 -cis form is never competitive, while 4 -trans is as competitive as the 3 -cis form for coverages below 0.15 ML [11]. Taking into account these previous studies, we analyzed the adsorption of cis and trans conformers of acrolein on Pt (1 1 1) through seven different bonding configurations: 1 -trans, 2 -cis, 2 -trans, 3 -cis, 3 -trans, 4 -cis and 4 -trans. The Pt (1 1 1) surface is modeled by a slab containing five atomic Pt layers with an acrolein molecule adsorbed only on one side of the slab. A low coverage of 1/9 ML has been considered in all cases. The gas-phase ˚ acrolein has been optimized in a periodic box of 13 A˚ × 13 A˚ × 20 A.

S. Pirillo et al. / Applied Surface Science 263 (2012) 79–85

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Fig. 1. GGA–DFT optimized adsorption geometries of the different interaction modes between acrolein and the Pt (1 1 1) surface. (a) 1 -trans-acrolein, (b) 2 trans-acrolein, (c) 3 -trans-acrolein, (d) 4 -trans-acrolein, (e) 2 -cis-acrolein, (f) 3 -cis-acrolein and (g) 4 -cis-acrolein. The utilized atom numbering is indicated.

3. Results and discussion 3.1. Bonding analysis We found that in all cases our interatomic lengths and bond angles are very similar to those reported by Loffreda et al. [11]. Then, we examined the evolution of the chemical bonding in these different preferential geometries, by means of COOP and OP analysis belonging to selected pairs of atoms. Table 1 shows the OP values corresponding to the bonds that participate most during the adsorption processes. Fig. 1 shows the seven optimized acrolein/Pt (1 1 1) structures and identifies the different atoms. It can be seen in Table 1 that the OP value corresponding to the C C bond is weakened between 31.2 and 33.9% during the adsorption acrolein in the 2 -cis, 2 -trans, 3 -cis, 3 -trans and 4 -trans modes, so a strong interaction between the mentioned bond and the surface occurs in these cases. This behavior can also be seen in Fig. 2, where the COOP curves belonging to the C C bond for the 2 , 3 and 4 -cis modes are compared to the isolated cis acrolein. As can be seen, the acrolein s and p states present narrow peaks in the isolated state, but once the adsorption occurs they re-hybridize as a result of the interaction with the surface d states and originate a dispersed band, in the range −2.0 to −6.0 eV. The COOP curves belonging to the trans isomers (not shown here) are very similar to those obtained for the 2 , 3 and 4 -cis forms. Table 1 reveals that the OP value of the C C bond for the 4 -cis configuration is only 22.0% lower than that belonging to the same bond in the isolated acrolein, thus anticipating a lower stability of this adsorption form with respect to the another 4 form and the 2 and 3 modes. In contrast, during the 1 -trans adsorption the C C overlap is weakened only 1.2%, since in this case adsorbate–surface interaction occurs only through the oxygen atom. In this adsorption mode the COOP curve corresponding to the C C is almost the same than that of the trans isomer in the isolated acrolein, as shown in Fig. 2. Regarding the C O bond, Table 1 shows that the OP value decreases 6.3% during the 2 -cis adsorption, 12.0% in the 3 -cis mode and 23.5% in the 4 -cis configuration, which evidences a growing interaction of the carbonyl group with the surface. In trans

Fig. 2. Crystal orbital overlap population (COOP) curves of C C bond of selected modes before and after adsorption process of acrolein on a Pt (1 1 1) surface. (a) 1 -trans, (b) 2 -cis, (c) 3 -cis and (d) 4 -cis.

modes we found a similar behavior, but in this case the adsorption geometry favors to a lesser extent such interaction and the C O overlap decreases 2.0, 6.2 and 20.0% for the 2 , 3 and 4 -trans configurations, respectively. Fig. 3 shows the COOP curves corresponding to the C O bond for the 2 , 3 and 4 -cis modes. The plots are similar, except for the presence of an antibonding peak which is located approximately at −4.3 eV in the curves belonging to 3 and 4 -cis adsorptions. This peak is more intense in the 4 configuration, matching the higher weakening of the C O bond observed during the OP analysis for this adsorption mode. In contrast, such antibonding peak does not appear in the curve belonging to the 2 configuration, thereby the weakening of this bond is smaller, as can be seen in Table 1. In the case of 1 -trans adsorption, a decrease of only 1.6% in the OP value of the C O bond is observed. Fig. 3 shows that the high strength of this bond in the 1 -trans interaction is due to the development of a noticeable bonding band between −6.0 and −4.5 eV. Table 1 presents that at the same time a strengthening of 2.0% of the C C overlap takes place in the 1 -trans configuration. This bond also strengthens in the 3 -cis mode, with an increase of 5.3% in the corresponding OP value. In the remaining configurations (2 -cis, 2 -trans, 3 -trans, 4 -cis and 4 -trans) that bond weakens, with changes from 0.6 to 3.1%. These trends in the strength of C C overlap are consistent with the elongation or contraction of this bond observed by Loffreda et al. [11] during the different adsorption modes, compared to isolated acrolein.

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Table 1 Overlap population (OP) before and after acrolein adsorption reaction.a Bond

Isolated acrolein cis

Pt Pt C C C C C O C Pt C Pt C Pt O Pt a

Pt (1 1 1)

trans 0.43

0.92 1.23 0.77

0.94 1.24 0.77

Acrolein/Pt (1 1 1) 1 -trans

2 -cis

2 -trans

3 -cis

3 -trans

4 -cis

4 -trans

0.41 0.95 1.22 0.76 0.00 0.00 0.00 0.21

0.34 0.91 0.83 0.72 0.03 0.34 0.49 0.00

0.34 0.91 0.84 0.75 0.08 0.34 0.49 0.00

0.34 0.97 0.83 0.68 0.09 0.32 0.50 0.16

0.34 0.92 0.82 0.72 0.08 0.34 0.50 0.07

0.35 0.89 0.96 0.59 0.15 0.19 0.33 0.13

0.31 0.91 0.85 0.61 0.19 0.26 0.49 0.16

Values in bold are those relevant to Section 3.

The OP values corresponding to the C Pt and O Pt bonds originated in the adsorption process are also presented in Table 1. OP values corresponding to H Pt bonds are not included because in all cases the overlaps are very low. It can be seen in Table 1 that, except for 1 configuration, adsorption of acrolein on the surface mainly occurs through the formation of C Pt bonds, whose OP values are more important in the case of the 2 and 3 modes. The C Pt overlap contributes less to the adsorption, though in the 4 configuration is more important. No C Pt overlaps are detected in the 1 geometry. Fig. 4 shows the COOP curves corresponding to the C Pt interaction in the 2 , 3 and 4 -cis forms. Similar curves were obtained for the 2 , 3 and 4 -trans configurations of the adsorbed acrolein

on Pt (1 1 1). While the plots in Fig. 4 are similar, an antibonding peak can be observed at approximately −1.5 eV, which is very pronounced in the 2 -cis configuration, some intense in the 3 -cis mode and virtually zero in the 2 -cis form. These differences in the COOP curves are consistent with the variations observed in the C Pt OP values listed in Table 1. At the same time, bonding states can be seen located below the Fermi level (Fig. 4), between −1.5 and −6.5 eV. In addition, Table 1 reveals that during the 3 and 4 adsorption modes the development of O Pt bonds also takes place, especially in 3 -cis and 4 -trans structures. This last overlap gives higher stability to those configurations. This difference can also be seen in Fig. 5, where the COOP curves for the O Pt bond formation are plotted. Indeed, the 3 -cis and 4 -trans modes have a very intense

Fig. 3. Crystal orbital overlap population (COOP) curves of C O bond of selected modes before and after adsorption process of acrolein on a Pt (1 1 1) surface. (a) 1 -trans, (b) 2 -cis, (c) 3 -cis and (d) 4 -cis.

Fig. 4. Crystal orbital overlap population (COOP) curves of C Pt formed bond of selected modes during adsorption process of acrolein on a Pt (1 1 1) surface. (a) 2 -cis, (b) 3 -cis and (c) 4 -cis.

S. Pirillo et al. / Applied Surface Science 263 (2012) 79–85

Fig. 5. Crystal orbital overlap population (COOP) curves of O Pt formed bond of selected modes during adsorption process of acrolein on a Pt (1 1 1) surface. (a) 1 -trans, (b) 3 -cis, (c) 3 -trans, (d) 4 -cis and (e) 4 -trans.

band between −4 and −6 eV, which is less developed in the 3 trans and 4 -cis forms. Note that in the 3 -cis mode this band is particularly intense, but at the same time antibonding interactions below the Fermi level are also present, which are more significant than in the other configurations (see Fig. 5). In the case of the 2 adsorption the oxygen atom does not interact with the surface, consequently no O Pt overlaps are possible. Contrary, in the 1 mode only the oxygen atom is involved in the interaction between acrolein and the surface, which originates a higher O Pt OP value than in the other analyzed configurations. Accordingly, it can be seen in Fig. 5 that the 1 -trans interaction originates a very intense bonding band between −4 and −6 eV and a bonding peak at lower energies (at −9.7 eV). This peak is less developed in the other adsorption geometries. The obtained OP values belonging to the formation of C Pt and O Pt bonds show that the most stable adsorption modes are 3 -cis and 4 -trans, followed by the 2 -cis and 2 -trans modes, then the 4 -cis form and finally the 1 -trans mode. This trend is consistent with the energetic study of Delbecq and Sautet [10]. Our bonding analysis also indicates that the 3 -trans mode is less stable than the most favorable cases. 3.2. Density of states The electronic structures of adsorbed 1 -trans-acrolein and 3 -cis-acrolein on the metallic surface and bare Pt (1 1 1) are presented in Fig. 6. The black tick marks show electronic levels of

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Fig. 6. Total DOS curves. (a) 1 -Trans acrolein on a Pt (1 1 1) surface, (b) 3 -cis acrolein on a Pt (1 1 1) surface and (c) bare Pt (1 1 1) surface. The tick marks in (a) and (b) indicate the molecular levels of cis and trans acrolein before adsorption, while the light blue shaded areas are the projections of adsorbed acrolein DOS.

isolated molecular species before adsorption. No adsorbate–adsorbate interaction was found. In the density of states curve of the chemisorbed molecule, it can be seen that the bands spread out and enlarged. These changes are the sign of the interactions with the metal orbitals. The dispersion corresponding to the bands between −7.5 and 6 eV indicates the strong interaction with the Pt surface, especially with the d-orbitals. Comparing 1 -trans-acrolein and 3 -cis-acrolein DOS curves we can see a major dispersion for 3 -cis-acrolein mode, which agrees to adsorption energy trend found by Delbecq and Sautet [10]. In the total DOS curves we can observe three peaks at −12, −15 and −22 eV for 3 -cis-acrolein, and at −12, −16 and −22 eV for 1 -trans-acrolein, corresponding to the interaction with the molecule s orbitals. The d-band width corresponding to the bare Pt (1 1 1) is approximately 8 eV, in agreement with ab-initio and semiempirical results reported in the literature [28–34]. Fig. 7 shows the orbital contributions to the DOS curves before and after the adsorption of 3 -cis acrolein, which is the most stable acrolein bonding configuration on Pt (1 1 1). In Fig. 7(a) we can see the DOS contributions corresponding to C p orbitals. When C p orbitals interact with the metallic surface, the DOS curve is extended over a 14 eV range. The contribution of O atom p orbital to the DOS curve is plotted in Fig. 7(b). The same behavior as the C p orbitals is observed. We can see a shift of approximately 0.75 eV after adsorption of Pt d orbitals (see Fig. 7(c)), while the d-band is pushed away from the Fermi level EF toward higher energies. At the same time, d-band is narrowed. These changes can be attributed to the interaction between the occupied ␲ orbitals and the almost full d-band of platinum, which stabilize the ␲ orbitals

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Table 2 Atomic orbital occupation for the acrolein/Pt (1 1 1) chemisorptions system.a Atom

C1 C3 H1 H2 H3 H4 O1 Pt Pt a

Orbital

2pz 2pz 1s 1s 1s 1s 2px 6pz 5dz2

Isolated acrolein cis

trans

0.40 0.79 1.00 0.97 0.97 0.97 1.97 – –

0.41 0.79 1.00 0.97 0.97 0.97 1.78 – –

Pt (1 1 1)

– – – – – – – 0.41 1.80

Acrolein/Pt (1 1 1) 1 -trans

2 -cis

2 -trans

3 -cis

3 -trans

4 -cis

4 -trans

0.62 0.93 0.99 0.97 0.97 0.97 1.92 0.41 1.80

0.56 1.00 0.94 0.94 0.98 0.98 1.90 0.30 1.30

0.54 0.99 0.94 0.94 0.98 0.98 1.73 0.32 1.29

0.63 0.99 0.97 0.94 0.98 0.98 1.87 0.31 1.27

0.57 1.00 0.96 0.94 0.98 0.98 1.72 0.32 1.29

0.68 0.88 0.97 0.97 0.97 0.97 1.92 0.30 1.19

0.76 1.00 0.95 0.94 0.98 0.98 1.68 0.28 1.31

Values in bold are those relevant to Section 3.

and destabilize the d-band. The crossing of part of the d-band above EF results in a stabilizing interaction. Fig. 7 also shows changes in the region between −7.5 and 6 eV, which means that C and O p orbitals strongly interact with Pt d orbitals, being responsible for the adsorption process. Pt, C, H and O s orbitals participation is not relevant. In order to get further insight about adsorbate–surface interactions, the contribution to chemisorption of the individual atomic orbitals of H, C, O and Pt atoms was studied. Table 2 shows corresponding atomic orbital occupations. Except for the 1 -trans mode, a significant decrease can be seen during the adsorption process (between 23.1 and 34.0%) in the pz and dz2 orbital occupation values of the interacting Pt atoms. In the case of the 1 -trans-acrolein,

the less favorable adsorption interaction, the Pt orbital occupations are almost the same than those obtained in the bare surface. At the same time, we can notice that the olefinic C pz population grows between 11.0 and 27.4%, depending on the adsorption mode, while the C1 pz orbital occupation increases more markedly, between 30.5 and 83.7%. For the energetically most favorable mode, the 3 cis acrolein, the C1 and C3 pz orbital occupation increase about 56.1 and 25.8%, respectively. Table 2 also shows that, except for the 1 -trans configuration, the O px orbital occupation decreases after adsorption, especially in the case of the most stable adsorption modes 3 -cis and 4 -trans, where the O Pt overlap plays an important role in the adsorbate–substrate interaction. In the case of the 1 -trans adsorption, the O atom interacts with the surface through its pz orbital, and a considerable decrease in its electronic population (approximately 13.2%) is achieved. The H s orbitals do not change significantly their occupation, so hydrogen atoms have a minor participation in the adsorption process. The adsorbate C pz orbital participates greatly in the adsorption process, as well as Pt pz and dz2 orbitals. In effect, the lobes of the C pz orbitals are perpendicular to the metal surface and are well oriented to overlap with the metal orbitals, especially with the pz and dz2 ones. 4. Conclusion

Fig. 7. Orbital contributions to DOS curves before and after 3 -cis acrolein adsorption on a Pt (1 1 1) surface. (a) C 2p orbital contribution, (b) O 2p orbital and (c) Pt 5d orbital contribution.

In this paper we examined by means of DFT and overlaps population analyses the evolution of the chemical bond during the adsorption of acrolein on a Pt (1 1 1) surface, considering the interaction modes proposed by Loffreda et al. [11]. We found for the different geometries very similar bond lengths and angles to those previously found. Through COOP and OP analysis we found that, except for the 1 configuration, adsorption of acrolein on the surface mainly occurs through the formation of C Pt bonds, whose OP values are more important in the case of the 2 and 3 modes. The C Pt overlap contributes less to the adsorption, although in the 4 configuration is more significant. The development of O Pt bonds also takes place during the 3 and 4 forms, especially in 3 -cis and 4 -trans structures. This last overlap provides greater stability to such configurations compared to the other studied adsorption modes. Our OP study seems to indicate that the most stable adsorption modes are 3 -cis and 4 trans, followed by the 2 -cis and 2 -trans modes. The 4 -cis and 1 -trans are the less favored adsorption structures, in agreement with the energetic results presented by Delbecq and Sautet [10]. The bonding analysis performed in our work also suggests that the 3 -trans mode (not reported by Delbecq and Sautet [10]) is a little less stable than the preferential ones. Finally, we found that the acrolein C pz orbital, perpendicular to the surface, participates notoriously in the adsorption, as well as Pt pz and dz2 orbitals, whose lobes are well oriented to overlap with the adsorbate orbitals.

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